The 5 th PSU-UNS International Conference on Engineering and Technology (ICET-2011), Phuket, May 2-3, 2011 Prince of Songkla University, Faculty of Engineering Hat Yai, Songkhla, Thailand 90112 Abstract: This paper is a proposal for an advanced approach for FEM modeling, structural analysis and design of a jib structure which is a typical part of the waterway bucket dredgers facilities. Dredgers with two catamaran-like pontoons, with jib in between, are considered here. The item of the analysis is the jib structure which will be re-constructed for the excavation of grain material from greater depths. Discussion here is oriented to the explanation of advantages and problems in the utilization of various FEM models. Key Words: FEM Modeling, Jib Structure, Optimal Design 1. INTRODUCTION In the classification of the waterway dredgers for the material exploitation under the water surface, a large group is bucket dredgers with the bucket on the continual chain supported by a jib structure, as is shown in Fig. 1. Fig. 1. Bucket dredger with the jib structure The excavation is performed by moving the bucket which plunges into the material at the water bed. Excavation continuity depends on the bucket size and span, as well as on the chain movement length and speed. This paper considers the design and FEM model for the jib structure as a girder for the dredger's working tool. The papers dealing with the FEM analysis of this type of structure (for example, [1]-[4]) are relatively rare. Their main feature is the application of complex models, but only to analyze the critical structural parts. This circumstance has additionally motivated the authors to research the alternatives, i.e. the possibilities for the advanced modeling of the jib structure as a whole. Due to the exploitation conditions, the jib in this type of dredgers should satisfy several opposed demands and its design should be, in the positive engineering sense, a compromise solution. Jib bearing capacity is the main and mandatory performance. Stress condition in all structural elements has to be within the boundaries which exclude the possibility of limit state reaching. Jib structure stability (i.e. buckling stiffness) is another requirement for the structure integrity. Global buckling is questionable, while the appearance of the local stability loss is possible since this is a thin-walled structure. Serviceability is the performance providing the conditions for the real exploitation service of the machine. It is normally connected to the stiffness, i.e. the state of displacement and deformation of the structure that provides the uninterrupted work with the possible failures only as a consequence of so-called "force majeure" circumstances. The above mentioned conditions are opposite to the demand for small mass of a jib structure because the mobility in exploitation and maintenance, as well as for the energy efficiency of the dredger. Furthermore, it is necessary the jib structure to be manufactured with the minimal quantity of steel and to be a simple design, regards to minimization of the manufacturing price. 2. THE JIB STRUCTURE POSSIBLE MODELS In the area of the applied structural design, the objective is to formulate the "optimal" model [5]. This is a model with the largest quality of the approximation achieved in the conditions of the "common designing practice", [6]. The choice of the model finally depends on the structure topology, action configuration and the assumed structure response. Waterway bucket dredger jib is a space structure with the notable length in comparison with other dimensions. It is a thin-walled structure with variable wall thickness, with the lateral stiffeners that increases the bearing capacity and stiffness of the structure. For the preliminary analyses, the satisfactory FEM models are the beam FE models. When the application of DESIGN CONCEPT OF A WATERWAY DREDGER JIB STRUCTURE D. Kovacevic*, I. Budak, A. Antic University of Novi Sad, Faculty of Technical Sciences, Novi Sad, Serbia *Authors to correspondence should be addressed via email: [email protected]239
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The 5th PSU-UNS International Conference on Engineering and
Technology (ICET-2011), Phuket, May 2-3, 2011 Prince of Songkla University, Faculty of Engineering
Hat Yai, Songkhla, Thailand 90112
Abstract: This paper is a proposal for an advanced
approach for FEM modeling, structural analysis and
design of a jib structure which is a typical part of the
waterway bucket dredgers facilities. Dredgers with two
catamaran-like pontoons, with jib in between, are
considered here. The item of the analysis is the jib
structure which will be re-constructed for the excavation
of grain material from greater depths. Discussion here is
oriented to the explanation of advantages and problems
in the utilization of various FEM models.
Key Words: FEM Modeling, Jib Structure, Optimal
Design
1. INTRODUCTION
In the classification of the waterway dredgers for the
material exploitation under the water surface, a large
group is bucket dredgers with the bucket on the continual
chain supported by a jib structure, as is shown in Fig. 1.
Fig. 1. Bucket dredger with the jib structure
The excavation is performed by moving the bucket
which plunges into the material at the water bed.
Excavation continuity depends on the bucket size and
span, as well as on the chain movement length and
speed.
This paper considers the design and FEM model for
the jib structure as a girder for the dredger's working
tool.
The papers dealing with the FEM analysis of this
type of structure (for example, [1]-[4]) are relatively rare.
Their main feature is the application of complex models,
but only to analyze the critical structural parts. This
circumstance has additionally motivated the authors to
research the alternatives, i.e. the possibilities for the
advanced modeling of the jib structure as a whole.
Due to the exploitation conditions, the jib in this type
of dredgers should satisfy several opposed demands and
its design should be, in the positive engineering sense, a
compromise solution. Jib bearing capacity is the main
and mandatory performance. Stress condition in all
structural elements has to be within the boundaries which
exclude the possibility of limit state reaching.
Jib structure stability (i.e. buckling stiffness) is
another requirement for the structure integrity. Global
buckling is questionable, while the appearance of the
local stability loss is possible since this is a thin-walled
structure.
Serviceability is the performance providing the
conditions for the real exploitation service of the
machine. It is normally connected to the stiffness, i.e. the
state of displacement and deformation of the structure
that provides the uninterrupted work with the possible
failures only as a consequence of so-called "force
majeure" circumstances.
The above mentioned conditions are opposite to the
demand for small mass of a jib structure because the
mobility in exploitation and maintenance, as well as for
the energy efficiency of the dredger. Furthermore, it is
necessary the jib structure to be manufactured with the
minimal quantity of steel and to be a simple design,
regards to minimization of the manufacturing price.
2. THE JIB STRUCTURE POSSIBLE MODELS
In the area of the applied structural design, the
objective is to formulate the "optimal" model [5]. This is
a model with the largest quality of the approximation
achieved in the conditions of the "common designing
practice", [6].
The choice of the model finally depends on the
structure topology, action configuration and the assumed
structure response. Waterway bucket dredger jib is a
space structure with the notable length in comparison
with other dimensions. It is a thin-walled structure with
variable wall thickness, with the lateral stiffeners that
increases the bearing capacity and stiffness of the
structure.
For the preliminary analyses, the satisfactory FEM
models are the beam FE models. When the application of
DESIGN CONCEPT OF A WATERWAY
DREDGER JIB STRUCTURE
D. Kovacevic*, I. Budak, A. Antic University of Novi Sad, Faculty of Technical Sciences, Novi Sad, Serbia
*Authors to correspondence should be addressed via email: [email protected]
239
these 1D models determines the approximate element
dimensions, they are followed by the sophisticated
analysis by applying the 2D FEM models with the
surface FE.
These 2D models almost completely satisfy the
demands of the developmental research, so they can be
the final models for the jib structural systems. The state
of stress and deformation in the jib thin-walled structure
can be approximated rather well by a model in which the
stresses in the normal direction on the plate surface that
present the jib cover are neglected. Furthermore, it is
reasonable to assume that there is no shear in the plate
mid-plane. These circumstances indicate the possibility
to utilize the model based on the Kirchhoff''s flexural
theory of thin plates, [5]. Exceptionally, for the plates
with relatively large thickness it is necessary to apply
the Reissner-Mindlin models for thick plate bending. If
one can avoid the appearance of the so-called "shear-
locking" phenomenon, the thick plate models that
consider the shear influence (i.e. real shear stiffness) can
provide very satisfactory results. Finally, if there is the
stress concentration in the local zones ("hot spot area"),
when the external action is distributed on a relatively
small surface, or if the stresses orthogonal to the plate
mid-plane cannot be neglected, the application of a 3D
model is an imperative.
A simple numerical test will illustrate the advantages
of a model with 2D FE in relation to the 1D FE model.
The results of this test could be main argument in the
final model choice. This and similar, "benchmark test"
should become an obligatory part in the model choice
methodology.
The analysis is performed for the vertical uniformly
distributed load equal to the weight of the chain with the
buckets. Fig. 2 presents 1D (top) and 2D (botom) models
and principal stress values, vertical displacements in
characteristic points and the lowest natural frequencies
for both models.
Fig. 2. 1D and 2D models: displacements principal
stresses and the lowest natural frequencies
1D model is formed from the beam FE (□240x
100x20mm box shape) and in the topological sense it is